The present invention concerns an electrochemical sensor for the determination of the concentration of hydrogen, carbon monoxide and silane, fluorine, bromine, iodine, oxygen, sulfur dioxide, methane, ethane, ethylene, acetylene and other gases. This sensor can inter alia be used for manufacturing portable, self-powered and easily operated instruments to measure and monitor the gas concentration.
Gas analyzers for continuously monitoring the surrounding medium are widely used in many fields such as in the automatic control of technological processes, explosion protection, ecological monitoring etc.. The construction of such analyzers may be based on electrochemical sensors. Various types of sensors are known for measuring and monitoring the gas concentration.
A known type of sensor (N. I. Globa: "Razrabotka i issledovanie electrochimicheskich datchikov konzentratsii kisloroda i vodoroda", author's dissertation paper Leningrad Institute of Technology, 1985, p. 11-14) contains a measuring electrode and a counterelectrode which are located in a liquid electrolyte. The measuring electrode is completely or partially composed of a catalytically active material. The counterelectrode is manufactured from an electrochemically active material, the selection of this material being dependent on the gas to be determined. If the material has low electrical conductivity, the counterelectrode is manufactured from a mixture of this material and carbon, in which case the carbon increases the electrical conductivity. The counterelectrode in an oxygen sensor is for example made of lead and in a hydrogen sensor it is made of a mixture of manganese dioxide and carbon.
When a gas the concentration of which is to be determined, is fed to the sensor an electrochemical oxidation (in the case of lead) or reduction (in the case of manganese dioxide) of the electrochemically active counterelectrode material takes place. This system which is composed of 2 electrodes generates an electrical current in an outer conductive circuit the magnitude of which is proportional to the gas concentration. This current can be utilized as a measure for the gas concentration.
The active material (lead or manganese dioxide) is consumed in the chemical reactions on the counterelectrode which limits the life of the sensor.
Passivation of the surface of the counterelectrode by products of the chemical reactions that take place on the electrode and diffusion of reaction products towards the measuring electrode can lead to errors in the measured signals which consequently decreases the reliability of the sensor. In order to reduce these effects, sensors which operate according to the principle stated above must be made with relatively large dimensions and large amounts of material.
A further known sensor for measuring the gas concentration (JP-A-59-28358) contains a measuring electrode made of a catalytically active material, an electrolyte and a counterelectrode which is composed of a mixture of carbon and an electrochemically active organic substance such as chloroquinone or monomeric and polymeric iron and cobalt phthalocyanine. The electrochemically active substance acts as a catalyst in the electrochemical reduction of oxygen.
When the gas to be measured is fed to the sensor it is oxidized on the measuring electrode. Accordingly a reduction of atmospheric oxygen or of specially supplied oxygen takes place on the counterelectrode which is made possible by the active components (catalysts). The catalysts are alternately oxidized and reduced during sensor operation. However, these two reactions are not absolutely reversible which leads to consumption of the catalysts and limits the life of the sensor. Moreover sensor reliability is only poor since the surface of the counterelectrode may be passivated by the products of oxygen reduction and these reaction products may diffuse towards the measuring electrode. A further disadvantage of the sensor is that the liquid electrolyte may dry out. However, the use of a solid electrolyte in such a sensor is also very difficult due to the need to generate a four-phase boundary "carbon-catalyst-electrolyte-oxygen".
Another restriction of using sensors of the above kind is that they can only operate for longer periods while being fed with oxygen i.e. in oxygen-containing media or with a specially implemented oxygen feed. Furthermore the use of liquid electrolytes results in a low mechanical strength of the sensor.
A sensor developed earlier by the same inventor contains a measuring electrode made of a catalytically active material, an electrolyte and a counterelectrode made of chemically pure carbon having a specific surface of 1,000 to 1,700 m.sup.2 /g. When the sensor is contacted with a gas the concentration of which is to be measured, the gas is electrochemically ionized at the measuring electrode. A charging process of the electrical double layer at the carbon-electrolyte boundary takes place at the counterelectrode. The resulting current measured in an external conductive circuit is proportional to the gas concentration and is used as a measure of the concentration.
Since the counterelectrode does not itself contain any electrochemically active components the life of the sensor is determined by the charging time of the electric double layer. An adequately long life can only be achieved when using a chemically pure carbon with a high specific surface (larger than 1,000 m.sup.2 /g). However, in this case the life of such a sensor with a size suitable for portable instruments is also only about 2 years.
Furthermore the manufacture of chemically pure carbon for the counterelectrode is difficult because oxygenous compounds are formed at the surface during the synthesis and activation of carbon. Due to the high adsorptivity of carbon it is practically impossible to prevent interactions between the electrolyte components and the chemically pure carbon. In turn such interactions can lead to a change of the potential of the counterelectrode and falsify the signals.